This invention relates to a process for the coating of substrates, e.g. metal foils, with polypropylene.
Laminates based on polypropylene and aluminium foil are becoming increasingly important. Compared with polyethylenes, polypropylene provides higher stiffness and tensile strength, a higher melting point (160.degree. C.) and very good resistance to attack by chemicals. For its part aluminium provides an outstanding barrier to gases, moisture and light, does not rust and can be readily shaped into functional products.
Hitherto, the limit to the extended use of processes for laminating polypropylene and metal films such as aluminium has been the relatively poor adhesion of polypropylene to metals for low cost high speed, extrusion coating processes. The chemically modified polyolefins commercially available can adhere to both polypropylene and metals by heat lamination processes. However to develop adequate adhesion the contact time at temperatures in the region of 200.degree. C. has to be at least 20 seconds. This excludes the use of the high speed extrusion coating process where although application temperatures are high, the contact time in the cooling nip is usually a fraction of a second when operating at economic running rates of at least 100 m/min of web.
Various patents have described the coating of substrates such as metal foil with polypropylene, e.g. U.S. Pat. Nos. 4,313,996 and 3,721,597, but none of them teach processes where high speeds of coating of over 50 m/min are possible. Also there is no evidence that good adhesion of the polypropylene to the substrate is achieved and no teaching of the necessity for having certain minimum quantities of acid or anhydride groups and ester groups in the terpolymer.
GB No. 1214820 discloses the use of terpolymers of ethylene and acrylic acid and vinyl acetate in which the total of the non-ethylene monomer is from 20-30 wt. % for adhering polyolefins including polypropylene to vinylidene chloride polymers. Coextrusion is used in the examples. GB No. 1263218, GB No. 1405769, EP No. 2240 discuss the combination of sheets of materials by pressing and heating. DE No. 1669685 disclose how sheets of polypropylene (Example 10) can be joined to steel plate using a sheet made of a mixture of polypropylene and a terpolymer.
There is no disclosure of how a thin layer of tie resin could be coextruded at high speed with polypropylene to provide a strong bond to substrates such as aluminium at high operating speeds. We have now devised a process which enables substrates e.g. metal foil, to be coated with polypropylene at high speed and with good adhesion.